1. Field of the Invention
This invention relates generally to grafted polymeric products, adhesive blends containing such products, and composite structures made therefrom and, more particularly, the invention relates to graft reaction products of propylene polymers and carboxylic acids and derivatives thereof, adhesive blends containing such grafted polymeric products, and composite structures incorporating such adhesive blends.
2. Description of Related Technology
Grafted polymeric products (e.g. graft copolymers) having polyolefin backbones grafted with polymerizable, ethylenically unsaturated carboxylic acids or acid derivatives, and the use of such products in adhesive blends, are well known. Various polyolefin backbones, such as ethylene homopolymers and copolymers and various forms of polypropylene have been utilized in the past.
Polypropylene-based graft copolymers are particularly useful in adhesive blends used for adhesion to one or more polypropylene substrates. Typically in the past such adhesive blends included a graft copolymer such as one having a polypropylene homopolymer backbone grafted with an acid anhydride such as maleic anhydride blended with a propylene homopolymer or an ethylene/propylene copolymer and, optionally, a third component such a linear low density polyethylene (LLDPE), a hydrocarbon rubber such as ethylene-propylene-diene monomer (EPDM) rubber or ethylene-propylene rubber (EPR), a poly(1-olefin) such as poly(butene-1), or an ethylene polymer such as high molecular weight low density polyethylene (HMW LDPE).
Prior polypropylene-based graft copolymers have been limited in terms of their maximum attainable functionality (i.e. concentration of grafted acid or derivative) and/or exhibited unacceptably low viscosity (as measured by high melt flow rates).
High acid or acid derivative functionalities are desirable in order to obtain desirable levels of adhesion at desirably low graft copolymer concentrations. Due to the inherently difficult process of grafting an acid or derivative to a polypropylene backbone, prior attempts to graft polypropylene, whether by the high temperature "thermal grafting" technique or by solution grafting methods resulted in maximum grafted acid or derivative concentrations of no more than about 1 to about 1.2 wt. %. Grafting in the presence of a peroxide or other free radical generating catalyst invariably resulted in chain scission and the resulting production of relatively low molecular weight grafted and ungrafted backbone fragments that must be removed from the grafting reaction product, as by solvent extraction, resulting in the loss of grafting monomer from the system.
For example, prior attempts to graft polypropylene homopolymer with maleic anhydride at a target grafted anhydride concentration of 2.0 wt. % typically resulted in about 1.5 to about 2.2 wt. % grafted monomer in the reaction product leaving the grafting reactor. After refining by solvent extraction to remove low molecular weight polymer fragments, the product would contain about 0.6 to about 1.2 wt. % grafted monomer.
Furthermore, prior attempts to graft polypropylene backbones with acid or derivative monomers resulted in unacceptably large increases in the melt flow rate of the product as compared to that of the ungrafted polypropylene, principally due to chain scission. For example, polypropylene homopolymer backbones having an initial melt flow rate (MFR) of about 2 to about 5 g/10 min. before grafting typically exhibited MFR values on the order of about 1500 g/10 min. after grafting with 1 wt. % maleic anhydride. Such melt flow rates are far too high for economical pelletizing operations such as the preferred underwater pelletizing method.
Thus, in the past, it was not possible to prepare highly functional polypropylene graft copolymers having desirably low melt flow rates. It is desirable to eliminate costly refining steps such as solvent extraction and to provide a graft polymeric product having a sufficiently low melt flow rate to facilitate pelletizing operations.